check-py3-compat: provide filename to ast.parse()

This ensures any warning/error messages print a valid filename instead of
potentially '<unknown>'.

Differential Revision: https://phab.mercurial-scm.org/D5844

contrib/check-py3-compat.py

@@ -45,7 +45,7 b' def check_compat_py3(f):'
         content = fh.read()
 
     try:
-        ast.parse(content)
+        ast.parse(content, filename=f)
     except SyntaxError as e:
         print('%s: invalid syntax: %s' % (f, e))
         return